376 THE TISSUES. 



The teeth are affected by contact, by heat, cold, and chemical 

 agents ; their sensibility arising from the nerves in their pulp. It 

 is quite delicate on the masticatory surfaces, where the smallest 

 foreign bodies as small grains of sand, &c. are at once perceived 

 when those surfaces are opposed to each other. It may also in dis- 

 ease become excessively acute. 



Slight mechanical influences can only act by the vibration which 

 they produce. Yet the teeth have a certain sense of locality, since 

 we can distinguish whether they are touched internally or exter- 

 nally, above or below, on the right or the left side. Acids cannot 

 penetrate the enamel, though it is not impermeable; since the nerves 

 of the pulp are not affected by them while the enamel is entire, but 

 are so at once when, as in the incisors, the dentine is exposed. 

 Nasmyth's membrane is doubtless still more impenetrable than the 

 enamel itself. 



Development of the Teeth. 



The first set (deciduous or milk-teeth) contains twenty, and the 

 second (permanent teeth) thirty-two teeth. Each tooth, during its 

 development, presents a papillary, a follicular, a saccular, and finally 

 its eruptive state. 



The development of the milk-teeth commences in the sixth 

 week of foetal life; twenty dental papillae making their appearance 

 from this period up to the tenth week, in a groove called the dental 

 groove. Next, partitions are formed between the papilla, and each 

 then lies in a special follicle or cavity ; and thus the papillary has 

 merged into the following stage. During the fourth month these 

 cavities contract, and finally close up completely, the papilla within 

 them at the same time assuming the forms of the future teeth ; and 

 thus the saccular stage is arrived at. A little cavity is, however, at 

 the same time prolonged from each closed tooth-sac ; these being 

 the "reserve-sacs" in which, during the fifth month, are developed 

 the pulps of the twenty anterior teeth of the second set. These 

 reserve-sacs, however, gradually retract backwards, and fall into 

 hollows in the jaw-bones; and lie at considerable depth in the latter 

 by the time the first set make their appearance by having attained 

 to the final or eruptive stage. 



The four stages just mentioned, and the relations of the "reserve- 

 sacs," are shown by Fig. ^37. The last are produced at their apices 

 into a solid cord, which has erroneously been called the gubernacu- 

 lum dentiSj or guiding cord for the permanent teeth in their eruption. 



